A triarylsulfonim salt (PTL 1), a phenacylsulfonium salt having a naphthalene skeleton (PTL 2), a dialkylbenzylsulfonium salt (PTL 3), a sulfonium salt having a thioxanthone skeleton introduced therein (PTL 4) and a sulfonium salt having a polyaryl sulfide skeleton (PTL 5) are conventionally known as a photoacid generator used in curing a cationically polymerizable compound such as an epoxy compound under the action of active energy rays such as light, electronic beams or X-rays.